An example of Pinnularia lanceolata var. similis prepared by RI Firth. This is the smaller vertically orientated one on the slide. Olympus BHB microscope using 450nm LED light. 63x Leitz Pl Apo NA 1.40 objective, oil immersion. Olympus Aplanat Achromat condenser, oil immersion, oblique lighting. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. 16 images stacked in Zerene (Pmax).
This is referenced in Cleve-Euler A. 1953. Die Diatomeen von Schweden und Finnland. Kungl. Svenska Vetensk.-Akad. Handl. 5:4 : 1–225. p.21 and Fig 1011b. A ‘rough’ Google translate of the relevant sections is given below.
“P. lanceolata n. sp. Shells narrowly lanceolate, gradually narrowing towards the pointedly rounded to almost pointed ends. Ash field (hyaline area?) distinct ± narrow, slightly expanded in the middle or a transverse band reaching the edges suddenly merging. Streaks strongly radial or convergent at the ends, (8.5) 10-13 by 10 µm. Length (34) 44-53 µm, width 7-10 µm.
β similis n. nom. P. viridis v. commutata Grun. small form, A. Cleve 1934 D. Finn Lapl. p. 46, Fig. 62. Streaks more densely spaced, 12-13; striations uninterrupted, moderately radiating. Ash field (hyaline area?) wider. Length 45 µm, width 7.5 µm. (Fig. 1011 b.)
Streams and lakes, here and there. Sm. Stråken (E.N.)! Annual Handöl! Lule Lpm. Aktse! Finn. Lappl. Vaskojoki-Inari! Kuolaj.!”
The one on the slide is 46.0µm long and 7.3µm wide, which fits with the description above.